This application claims the benefit of Korean Patent Application Nos. 2002-9288 and 2003-9495 filed on Feb. 21, 2002 and on Feb. 14, 2003 respectively, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a mobile communication apparatus including an antenna array, by which interference of a received signal between users can be removed by massing the beams of a signal to be transmitted to individual users in order to minimize the influence of fading, interference, and noise in a mobile communication environment, and a mobile communication method performed in the mobile communication apparatus, and more particularly, to a mobile communication apparatus capable of improving the performance of a mobile communication system by reducing the amount of feedback information in order to make the mobile communication system insensitive to feedback errors or feedback delays, in contrast with an eigen beamforming method proposed by Siemens, in which eigenvectors are directly,quantized, and the quantized eigenvectors are fed by a mobile station to a base station, and a mobile communication method performed in the mobile communication apparatus.
2. Description of the Related Art
A next-generation mobile communication system is required to transmit information faster than existing mobile communication systems such as personal communication services (PCSs). Europe and Japan have already adopted a wideband code division multiple access (W-CDMA) system as a wireless access standard, while the North America has already adopted a CDMA-2000 (multi-carrier code division multiple access) system.
In a general mobile communication system, several mobile stations communicate with one another through a base station. In order to transmit data at a high speed, a mobile communication system should minimize loss due to the characteristics of a mobile communication channel, such as fading, and user interference. In particular, diversity systems are used to prevent communication from becoming unstable due to fading. A space diversity system, which is a type of diversity system, uses multiple antennas, that is, an antenna array.
In order to achieve fast data transmission, a general mobile communication system should overcome fading having the most serious effect on the performance of the mobile communication system, because fading reduces the amplitude of a received signal to several dB or several tens of dB. Here, the fading is one of the channel characteristics of the mobile communication system. As described above, fading can be overcome by several diversity techniques. A representative example of the diversity techniques is a rake receiver for performing the diversity using the delay or spread of a channel in a CDMA technique. A rake receiver performs a diversity reception technique for receiving a multi-path signal. However, the diversity technique does not operate diversity when a delay spread is low.
Another example of the diversity techniques is a time diversity system using interleaving and coding, which is used in a Doppler spread channel. However, the time diversity system is not suitable for a low-speed Doppler channel. In a room channel with a low delay spread and a pedestrian channel corresponding to a low Doppler channel, a space diversity system is used in order to overcome fading. A space diversity system uses at least two antennas. If a signal received via one antenna is attenuated by fading, the space diversity system receives the signal via another antenna. The space diversity is classified into a reception antenna diversity using a reception antenna and a transmission antenna diversity using a transmission antenna. As it is difficult for a mobile station to install the reception antenna diversity in respect of area and costs, it is recommended that a base station use the transmission antenna diversity.
In the transmission antenna diversity, there are a closed loop transmission antenna diversity getting feedback of a downlink channel information from a mobile station to the base station, and an open loop transmission antenna diversity getting no feedback from a mobile station to the base station. In the transmission antenna diversity, a mobile station searches for an optimal weighted value by measuring the phase and magnitude of a downward channel formed from a base station to a mobile station and transmits the searched information to the base station. In order to measure the magnitude and phase of the moving channel, a base station must send different orthogonal pilot signals for different transmission antennas. A mobile station receives the pilot signals, measures the magnitude and phase of a channel using the received pilot signals, and searches for an optimal weighted value for a transmission antenna diversity from the measured channel magnitude and phase information.
For the transmission antenna diversity, if the number of transmission antennas of the base station increases, the diversity effect and the signal-to-noise ratio still improve, but the amount/speed of improvement in the diversity effect continuously decreases. Accordingly, to obtain a slightly-improved diversity effect while sacrificing a lot cost is not preferable. Hence, it is preferable that the number of antennas used in a base station increases to minimize the power of an interference signal and maximize the signal-to-noise ratio of an internal signal, instead of improving the diversity effect.
A transmission adaptive antenna array system invented in consideration of a beamforming effect that minimizes the influence that interference and noise as well as diversity effect have upon an internal signal is referred to as a downlink beamforming system. A system using feedback information like a transmission diversity is referred to as a closed loop downlink beamforming system. The closed loop downlink beamforming system, which uses information fed back from a mobile station to a base station, may degrade the performance of communications by failing to properly reflect changes in channel information if a feedback channel does not have a sufficient bandwidth.
The first and second TxAA modes standardized in a W-CDMA system, which is a European IMT-2000have the following problems when the number of antennas and the characteristics of a space-time channel vary. If the number of antennas increases, a weighted value for each antenna must be fed back, and hence a lot of information to be fed back is created. Thus, depending on the movement speed of a mobile station, the first and second TxAA modes degrade the communication performance. That is, generally, if the movement speed of a mobile station increases in a fading channel, a change in the space-time channel becomes serious. Thus, the feedback speed of channel information must increase. However, if the feedback speed is limited, feedback information increasing with an increase in the number of antennas consequently degrades the performance of communications. If the distance between antennas is not sufficient, the correlation between channels in each antenna increases. If the correlation between channels increases, the information amount of a channel matrix decreases. The effective use of a feedback method prevents performance degradation in a high-speed moving body environment even if the number of antennas increases. However, since the first and second TxAA modes are constructed under the assumption that the channels of two antennas that constitute the space-time channels are completely independent from each other, they cannot be used effectively when the number of antennas and the characteristics of the space-time channel change. In addition, the first and second TxAA modes have never been applied to an environment using more than 2 antennas and cannot provide excellent performance even when using 3 or more antennas.
Because of the above reasons, a beamforming antenna system is formed in case three or more antennas are used. A beamforming technique uses the difference in a direction between individual users and is suitable for an environment having a great correlation between channels of individual transceiving antennas. In particular, Siemens suggests that the 3GPP adopts an eigen beamforming technique which combines a diversity with a beam-forming. However, the eigen beamforming technique provides a lot of feedback information because it simply quantizes eigenvectors for beamforming and feeds the quantized eigenvectors back to a sender. Such a feedback of information in a large amount causes feedback information during transmission to be sensitive to generated errors and delays, thereby degrading the performance of mobile communication systems.